Polysaccharides such as agaroses, dextrans and cyclodextrans are widely used materials in the life science and biology fields. They can be used as substrates for electrophoresis or as a capture or chromatography media, either directly as a size exclusion material or through the bonding of various capture ligands, such as Protein A to their surfaces or pores.
These products have for the most part been formed by a thermal phase separation process that separates the polymer from an aqueous phase. This is done because these polymers have a melting point and a gel point.
According to the prior art, to make an aqueous solution of agarose, the polymer must be heated above its melting temperature, which is about 92° C., in the presence of water. At or above that temperature the polymer melts and the molten polymer is then solvated by the water. The polymer remains soluble in water as long as the temperature is above the polymer's gel point, which is about 43° C. At and below the gel point, the polymer phase separates and becomes a hydrogel that takes on whatever shape the solution was in just before gelling.
An example of this process is the method for making agarose gel beads as illustrated in U.S. Pat. No. 5,723,601. In this process, an agarose solution is prepared by heating a mixture of agarose and water above the melting point of the polymer; the solution is then maintained at a temperature between the gel point and the melting temperature so that it can be processed. The hot agarose solution is then poured into a hot water-immiscible non-solvent for agarose (oil) and an emulsion is then prepared to form small droplets of aqueous agarose solution suspended in oil. Once the droplets have been formed, the entire system is cooled below the gel point of agarose to gel the droplets and thus form the gel beads.
The two main problems with the polysaccharide solutions of the prior are: (1) the need to maintain them at elevated temperatures without losing water thereby altering the composition of the solution and (2) the gelling behavior of the solution at lower temperatures thereby creating a gel of a fixed shape. This limits the range of applicability of these polymers to formats other than beads or slabs, such as coatings on porous materials due to the inability to process the polymer at room temperature to create layers (coatings) without substantially blocking the pores of said porous materials.
An alternative is known from WO 00/44928 in which agarose is dissolved in water with the use of one or more chaotropes such as urea, guanidium salts or potassium iodide. The solution of agarose formed in this way does not gel at room temperature. However, the coated porous structure that is made is not substantially porous. This may be due to the method of re-gelling the agarose or other factors not stated in the text.